Neurostimulation systems are devices that generate electrical pulses and deliver the pulses to nerve tissue to treat a variety of disorders. Spinal cord stimulation (SCS) is an example of neurostimulation in which electrical pulses are delivered to nerve tissue in the spine, typically for the purpose of treating chronic pain. While a precise understanding of the interaction between the applied electrical energy and the nervous tissue is not fully appreciated, it is known that application of an electrical field to spinal nervous tissue can effectively mask certain types of pain transmitted from regions of the body associated with the stimulated nerve tissue. Specifically, applying electrical energy to the spinal cord associated with regions of the body afflicted with chronic pain can induce “paresthesia” (a subjective sensation of numbness or tingling) in the afflicted bodily regions. Thereby, paresthesia can effectively mask the transmission of non-acute pain sensations to the brain.
Neurostimulation systems generally include a pulse generator and one or several leads. The pulse generator is typically implemented using a metallic housing that encloses circuitry for generating the electrical pulses. The pulse generator is usually implanted within a subcutaneous pocket created under the skin by a physician. The leads are used to conduct the electrical pulses from the implant site of the pulse generator to the targeted nerve tissue. The leads typically include a lead body of an insulative polymer material with embedded wire conductors extending through the lead body. Electrodes on a distal end of the lead body are coupled to the conductors to deliver the electrical pulses to the nerve tissue.
The effectiveness of a neurostimulation therapy is dependent upon recruiting the appropriate nerve fibers for stimulation while avoiding unrelated nerve fibers (e.g., the nerve fibers corresponding to areas without pain or corresponding to motor functions). Accordingly, it has been proposed to “steer” current in an attempt to localize the specific nerves that will be stimulated by the electrical pulses. As currently known in the art, there are two different methods of steering current. In one embodiment, pulse amplitude “fractionalization” or “tripolar” stimulation is applied. In such techniques, anodes of varying amplitudes are placed on either side of a cathode. The superposition of the fields created by the tripolar arrangement tend to laterally shape or steer the locus of stimulation. Another technique applies stimulation multiple pulses in a non-simultaneous manner using different electrode combinations. However, the pulses are provided within a sufficiently small amount of time that the combined effect of the multiple pulses tend to create a locus of stimulation that depends upon the relative amplitude of the pulses and the timing between the pulses.